The present invention relates to coating compositions that utilize acrylic polymers which can be dispersed or solubilized in water.
Generally speaking, there are two basic ways to synthesize a water reducible or dispersible polymer to be used for the preparation of aqueous coating compositions. In the first instance, a monomer mixture which contains some acid group containing monomer is polymerized such that the resultant polymer contains acid functionality. Acid functional acrylic polymers are an example of this type of material. Alternatively, one can condense mono-, di- or polybasic acids with polyols to form a polyester or alkyd resin in which residual acid functionality is present for the purposes of water dispersion or solubilization. These polymers are water soluble or dispersible when neutralized with an appropriate base such as an amine and an appropriate organic cosolvent. Coating compositions prepared from such polymers can be crosslinked with aminoplast type crosslinking agents which react primarily with the reactive hydroxyl and amide groups of the polymers. Therefore, after cure, the films which are formed from these water solubilized compositions typically contain unreacted carboxyl groups which were utilized in the water solubilization or dispersion process but which were not consumed during the curing process. Although, the art discloses systems where anionic carboxyl functional waterborne polymers have been cured with polyepoxides, polyaziridines or carbodiimides to consume the carboxyl functionality by which they are waterborne, these materials are relatively toxic and difficult to handle and necessitate two pack application for stability.
Another manner of preparing water reducible or water solubilized polymers for aqueous coating compositions is with the assistance of externally added surfactants which act as emulsifiers to facilitate the solubilization of the polymer into water even though water solubilizing groups are not necessarily present.
These two approaches to the preparation of water reducible or water solubilized materials share several advantages and disadvantages. The materials are advantageous because they permit the formulation of water based coating compositions which have typically a low volatile organic content and at the same time have a variety of good coating properties such as ease of application, good gloss and general appearance. The common disadvantage of the two approaches is the diminished water resistance of coating compositions formulated from the water dispersible polymer. In the first approach, water sensitivity is imparted to the coating from the residual unreacted acid groups which remain after cure of the coating composition. These groups adversely affect the coating's ability to withstand humid environments and also contribute to diminished corrosion resistance and adhesion of the coating. In the second instance, the surfactants which are utilized to assist in emulsification or solubilization of the polymer into water are themselves highly water soluble materials and hence contribute to the overall water sensitivity of coating films derived from such polymers. Once again, the presence of these materials in the film adversely affects the film's ability to withstand humid environments and also diminish the corrosion resistance properties of the film as well as the wet adhesion.
There is a need, therefore, for a waterborne polymer which is capable not only of being solubilized into water but also cured by curing agents such as aminoplast and blocked isocyanate crosslinking agents without the presence of acid functionality or externally added surfactants.